Current limiting polymer compositions which exhibit positive temperature coefficient of resistance (PTC) behavior, and electrical devices comprising current limiting polymer compositions have been widely used. The current limiting polymer compositions generally include conductive particles, such as carbon black, graphite or metal particles, dispersed in a polymer matrix, such as thermoplastic polymer, elastomeric polymer or thermosetting polymer. PTC behavior in a current limiting polymer composition is characterized by the material undergoing a sharp increase in resistivity as its temperature rises above a particular value otherwise known as the anomaly or switching temperature, T.sub.s. Materials exhibiting PTC behavior are useful in a number of applications including electrical circuit protection devices in which the current passing through a circuit is controlled by the temperature of a PTC element forming part of that circuit.
Particularly useful devices comprising current limiting polymer compositions are electrical circuit protection devices. Such circuit protection devices usually contain a current limiting polymer device comprised of two electrodes embedded in a current limiting polymer composition. When connected to a circuit, the circuit protection devices have a relatively low resistance under normal operating conditions of the circuit, but are tripped, that is, converted into a high resistance state when a fault condition, for example, excessive current or temperature, occurs. When the circuit protection device is tripped by excessive current, the current passing through the PTC device causes it to self-heat to its transition temperature or switching temperature, T.sub.s, at which a rapid increase in its resistance takes place, to transform it to a high resistance state.
Representative electrical circuit protection devices and current limiting polymer compositions for use in such devices are described, for example, in U.S. Pat. Nos. 4,545,926 (Fouts, Jr., et al.); 4,647,894 (Ratell); 4,685,025 (Carlomagno); 4,724,417 (Au, et al.); 4,774,024 (Deep, et al.); 4,775,778 (van Konynenburg, et al.); 4,857,880 (Au, et al.); 4,910,389 (Sherman, et al.); 5,049,850 (Evans); and 5,195,013 (Jacobs, et al.).
In such devices a current limiting polymer composition is attached in some manner to a source of electrical power. This is generally provided by what is referred to in the art as an electrode which is in contact with the current limiting polymer composition and which is connected to a source of electrical power. The interface in these devices between the current limiting polymer composition and the electrode presents certain problems which limit the range of applications in which such devices can be reliably implemented commercially. For example, the avoidance of excessive current concentrations at any spot near the electrodes of the device presents problems, as does the provision of electrodes in a form which will reliably distribute the current over a suitable cross-sectional area of the current limiting polymer composition of the device and without variations of such distribution on repeated cycles of operation of the device. Furthermore, the use of electrodes may lead to some degree of electrical non-uniformity; if the surface of the electrode closest to the other electrode has any imperfections, this can lead to an electrical stress concentration which will cause poor performance. This problem is particularly serious when the current limiting polymer composition exhibits PTC behavior, since it can cause creation of a hot zone adjacent to the electrode; it also becomes increasingly serious as the distance between the electrodes gets smaller. Moreover, in a current limiting polymer device having a total contact resistance that exceeds 20% to 45% of the overall device resistance will, depending on the thermal transfer characteristics of the current limiting polymer composition, initially begin to undergo transition at the interface and will exhibit an increased potential for arcing across the interface. Such arcing may cause damage to the polymer composition and thereby limit the life of the device. Specifically, arcing may result in electrode delamination or a thermal/electrical break down in the electrode/polymer composition interface.
Notwithstanding, current limiting polymer compositions have found commercial application in circuit protection devices for telecommunications lines and for surge protection in small motors. Such devices, however, have been limited to use in systems with relatively low currents and voltages. These devices have been so limited due, in part, to the level of contact resistance associated with the interface between the current limiting polymer composition and the electrodes. It has been determined that the contact resistance in these devices can contribute up to 75% of the total device resistance. Accordingly, it would be desirable to have an interface between the current limiting polymer composition and the electrodes that results in a low contact resistance for the device.
The electrodes which have been used in such current limiting PTC polymer devices include solid and stranded wires, wire rovings, metal foils, expanded metal, perforated metal sheets, etc. A variety of methods have been developed for connecting the electrodes to the current limiting polymer composition. For example, U.S. Pat. Nos. 3,351,882 (Kohler, et al.); 4,272,471 (Walker); 4,426,633 (Taylor); 4,314,231 (Walty); 4,689,475 (Kleiner, et al. '475); 4,800,253 (Kleiner, et al. '253); and 4,924,074 (Fang, et al.).
Specifically, Walty describes a method for attaching planer electrodes to current limiting polymer compositions using an electrically conductive adhesive. Taylor discloses a method for laminating metal foil electrodes to the current limiting polymer composition through the use of pressure, heat and time. Taylor also discloses the optional use of an electrically conductive adhesive to help bind the electrode to the current limiting polymer composition. Finally, Kleiner, et al. '253 & '475 disclose the use of electrodes with microrough surfaces. Namely, Kleiner, et al., teaches the use of electrodes that have a roughened surface obtained by removal of material from the surface of a smooth electrode, e.g. by etching; by chemical reaction on the surface of a smooth electrode, e.g. by galvanic deposition; or by deposition of a microrough layer of the same or a different material on the surface of the electrode.
In order to obtain room temperature resistance levels in the 0.1-5 m.OMEGA. range, low bulk resistivity and low contact resistance are necessary. Current limiting polymer composition based electrical devices having a voltage rating of 500 V.sub.rms and a current rating of 63 A.sub.rms steady state for reducing let-through values in molded case circuit breakers are available. To achieve these high voltage and current ratings, however, the currently available devices require a large area parallel plate geometry with high spring pressure to connect the electrodes to the current limiting polymer composition. The high spring pressure connecting the electrodes to the current limiting polymer composition helps to reduce the contact resistance. As the pressure increases the area of real contact between the electrode and the current limiting polymer composition increases. Also the area of contact by the electrode with the conductive filler increases with increasing pressure. At these elevated pressures, the current limiting polymer composition plastically deforms to make intimate contact with the electrodes. A thin layer of polymer may cover a large percentage of the contact area between the electrodes and the current limiting polymer composition. This thin layer of polymer will prevent direct contact between the conductive filler particles in the current limiting polymer composition and the electrodes. This factor limits the decrease in device resistance obtainable through the application of pressure to connect electrodes to the current limiting polymer composition. Furthermore, the resulting device requires a large package and consequently has to be mounted externally to the circuit breaker. Therefore, it would be desirable to have a method for attaching electrodes to current limiting polymer compositions which would provide for a compact geometry and which would not require high spring pressure.
What is needed are current limiting PTC polymer devices which have a low contact resistance capable of use in high current/high voltage applications. Particularly what is needed is a method for attaching electrodes to a current limiting polymer composition and for preparing the current limiting polymer composition for such attachment which results in a low resistance electrical interface relative to the overall device resistance. A low contact resistance relative to the overall device resistance is desirable for two main reasons. First, the joule heating will occur in the bulk of the current limiting polymer composition thus inhibiting arcing at the electrode/polymer interface. Such arcing often results in electrode delamination or a thermal/electrical break down in the electrode/polymer interface. Second, the lower the overall device resistance the higher the steady state current ratings obtainable for the device.